It seems there may be a misunderstanding regarding mortgages. For the secondary market (where mortgages are securitized and traded), the collateral for those loans is always real property. Manufactured homes can be either personal property or real property. If they're permanently attached to the ground, tongue & wheels removed, and title forfeited (generally speaking), the taxing authority and/or state will designate the manufactured home as 'real property' (now part of the land). Otherwise, manufactured homes are personal property.
In the world of appraisal, these are two types of specialization - real property appraisal and personal property appraisal. Real property appraisers are regulated by their respective states, all of which that I'm aware of have USPAP compliance baked into the real property appraiser regs. Personal property appraisers aren't regulated by the state and, as such, are not bound by regulation to comply with USPAP.
So in answer to your question, I'd change it a bit to say that for a manufactured home to be eligible for a mortgage, it must be part of the real estate. Otherwise, any debt secured against the home would simply be a loan.